1. Field of the Invention
The present invention relates to injectors and fuel nozzles for high temperature applications, and more particularly, to fuel injectors and nozzles for gas turbine engines used in aircraft.
2. Description of Related Art
A variety of devices and methods are known in the art for injecting fuel into gas turbine engines. Of such devices, many are directed to injecting fuel into combustors of gas turbine engines under high temperature conditions.
Gas turbines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. A common problem associated with fuel nozzles for gas turbines is the difficulty associated with performance at low fuel flows and/or air flow and pressure drop across the combustor. Prefilming airblast atomizers are a preferred approach for combustion systems operating at high pressures because they require lower fuel pump pressures and produce a well mixed finely atomized spray at standard operating conditions.
There are several technical problems associated with the combustion process in gas turbine engines. These problems include, for example, thermal efficiency of the burner/combustor, proper mixing of fuel and air, flame stabilization, the elimination of pulsations and noise, and the control of polluting emissions, especially nitrogen oxides (NOX). Flame stabilization refers to fixing the position and intensity of the flame within the burner so as to eliminate pulsations and reduce noise, among other things. Stable combustion particularly at lean extinction points in gas turbine engines requires a cyclic process of combustion producing products, i.e., heat and free radicals, which are transported back upstream to the flame initiation point to facilitate the combustion process.
It is presently known to provide swirled air to the fuel-air mixture in order to improve flame stabilization and thereby stabilize the combustion process. Swirl stabilized combustion flows facilitate combustion by developing reverse flow about the centerline of the burner, which returns heat and free radicals back upstream to the un-burnt fuel-air mixture.
U.S. Patent Application Publication No. 2005/0106520 to Cornwell, et al., which is incorporated herein by reference in its entirety, describes a device for stabilizing combustion in gas turbine engines. A central bluff body flame holder extends through a mixing chamber that includes a plurality of swirl vanes for swirling air. Fuel injected into the swirling flow is mixed with air for combustion downstream. The central bluff body cooperates with a quart to anchor the flame to the bluff body and stabilize the flame over a wide range of operating conditions, including fluctuating fuel/air ratios. This design provides for flexibility and robustness in engine operation, while maintaining low NOX emissions.
Another problem associated with gas turbines is the difficulty associated with initiating fuel ignition during engine startup cycles and, for aviation, altitude relights. During these startup cycles, the fuel must be presented in a sufficiently atomized condition to initiate and support ignition. However, at engine start up, when the engine is gradually spooling up, the fuel and/or air pressure needed to atomize the fuel is generally unavailable. A broad range of fuel injection devices and methods has been developed to enhance the fuel atomization during engine ignition sequences and lean extinction points. One approach has been to employ air assist or airblast atomizers to facilitate the atomization process.
An exemplary air assist atomizer for gas turbine engines is described in U.S. Pat. No. 6,688,534 to Bretz, which is incorporated by reference herein in its entirety. The fuel delivery system includes an inner and outer air swirler with a fuel injection orifice there between. Rotating streams of air from the inner and outer air swirlers surround and shear fuel issuing from the fuel injection orifices therebetween to atomize the fuel for combustion. The air assist atomizer utilizes an air assist circuit where the atomizing air during ignition is supplemented by a separate external source. The high pressure air assist air is routed through a set of vanes to induce swirl and merged with the low pressure compressor discharge air to enhance the fuel atomization process to facilitate ignition.
Another way of improving engine emissions and efficiency is by using staged combustion. An exemplary lean, direct injection atomizer for gas turbine engines is described in U.S. Patent Application Publication No. 2006/0248898 to Buelow et al., which is incorporated by reference herein in its entirety. The lean direct injection fuel nozzle includes an outer main fuel delivery system and an inner pilot fuel delivery system. Each of the main and pilot fuel delivery systems includes an inner and outer air swirler with a fuel injection orifice therebetween. Rotating streams of air from the inner and outer air swirlers surround and shear fuel issuing from the fuel injection orifices therebetween to atomize the fuel for combustion. During low power operation, only the pilot combustion zone is fueled, and during high power operation, both pilot and main combustion zones are fueled. The pilot combustion zone provides low power operation as well as good flame stability at high power operation. The main combustion zone operates in a fuel-lean mode for reduced flame temperature and low pollutant formation, particularly NOX. During high power operation, the ignition source for the main fuel-air mixture comes from the pilot combustion zone.
Another exemplary fuel injection nozzle is shown in FIG. 1. Nozzle 1 includes upstream end 2 and downstream end 3. Inner air swirler 4 and outer air swirler 5 add swirl to air passing through from a compressor upstream. This creates an expanding vortex where the air exits the fuel nozzle. The swirling air entrains fuel from fuel outlet 6 and the resulting volumetric expansion associated with vortex flow further strains the fuel sheet, shearing the fuel sheet into droplets. As the vortex progresses into the combustor downstream of nozzle 1, the pressure gradient becomes such that the vortex cannot sustain itself and it collapses. The vortex consequently recirculates along the low-pressure centerline of the vortex. Typically the vortex will have a tendency to be unsteady about its centerline. Pure air blast injectors of the prior art utilize a wake region from a center-body of an axial swirler, such as center-body 7, to help stabilize the centerline of the vortex. Alternatively, in the case of a radial in-flow swirler (not shown), a low-pressure region from the inlet has been used to help stabilize the centerline of the vortex.
Fuel is introduced via a prefilming chamber (upstream of fuel outlet 6) functioning to shear angularly injected fuel into a uniform sheet. This uniform sheet proceeds along the prefilming surface to a fuel exit annulus 6, at which point the fuel enters into a cross-flowing air stream. The cross-flowing air stream has much higher kinetic energy that interacts with and excites the low kinetic energy fuel sheet. This interaction shears and accelerates the fuel sheet, creating multiple modes of instability, which ultimately results in the fuel sheet breaking into ligaments of fuel. These fuel ligaments are similarly excited and broken into droplets. This is the primary mode of droplet formation, which requires the cross-flowing air stream to have enough energy to cause excitation. The more effective the energy transfer from air to fuel, the smaller the diameter of droplets, which inevitably results in a more uniform fuel/air mixture that more readily combusts given an adequate ratio of fuel to air. At lower power conditions however, there is less momentum in the compressor discharge air, leaving a deficiency in the ability to excite the fuel film which therefore results in droplets that are not sufficiently small.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a nozzle or fuel injector that allows for improved excitation of the fuel film at low power conditions. The present invention provides a solution for these problems.